This invention relates to a variable geometry combustion for a gas turbine engine and to a gas turbine engine provided with such a combustion.
Gas turbine engines in industrial applications are expected to operate over a range of varying load conditions rather than at some fixed optimum. It is also a requirement that certain minimum standards must be met in respect of environmental pollution from engine exhausts. In order to meet these demands, which are often in conflict, the combustion engineer is faced with substantial design difficulties. For example, in order to lower polluting NOx emissions, it is common to use so-called lean pre-mix systems which are effective during engine high load conditions. Unfortunately, such systems tend to increase polluting CO emissions at engine low load conditions (due to incomplete combustion at lower flame temperatures), and conventional methods of controlling CO emissions, such as air bleed systems, may result in loss of engine efficiency.
Attempts to overcome these difficulties include the use of what have become known as xe2x80x9cvariable geometry systemsxe2x80x9d (see ASME paper 95-GT-48 by Yamada, et al.), in which combustion system air (typically supplied from the engine compressor) is controlled so that, when an engine is being run at low load, proportionally less air is fed to the combustion chamber upstream fuel mixing region than is the case for higher loads. The balance of air required for the combustion system is diverted to a downstream region of the combustion chamber where it can do useful work in the gas stream. In this way the compressor and all compressor air is most effectively employed in contrast with other systems where the compressor output may be adjusted to give less flow, or where some of the compressed air is vented off (both such schemes usually being less efficient). Such a variable air distribution system allows flame temperatures to be held reasonably constant at the optimum design higher load level (higher temperature) and consequently pollution emission levels may be held to a minimum.
Mechanisms for controlling air distribution in xe2x80x9cvariable geometry systemsxe2x80x9d usually consist of connected valve means acting in unison to divert compressor air proportionally to upstream and downstream regions of a combustion chamber, the combustion chamber being fixed in position relative to the engine main casing, as can be seen, for example, in U.S. Pat. No. 3,859,787 to Anderson, et al.
On the other hand, U.K. Patent No. GB 1,160,709 to Lucas discloses an annular combustion comprising a combustion chamber or flame tube which is bodily movable axially within an air jacket casing or manifold. There are inlets for primary and secondary air in upstream and downstream regions of the flame tube, referred to the direction of flow of combustion products through the flame tube. Movement of the flame tube is towards or away from the upstream end of the combustion, an inlet for primary combustion air being defined between a fixed burner head and the upstream end of the movable flame tube. Hence, movement of the flame tube relative to the burner head varies the size of the primary inlet, but there is no provision for varying the size of the secondary inlet.
An object of the present invention is to provide a relatively simple, inexpensive, convenient and easily controlled way of metering primary and secondary flows of air into the upstream and downstream regions of a combustion chamber simultaneously and in proportions which facilitate efficient combustion at high- and low-load conditions of the engine.
The invention can achieve the above object by linear movement of a combustion component.
According to the invention, a gas turbine combustor comprises a combustion chamber mounted within an air supply manifold. The combustion chamber has a burner head provided with a fuel injector means; a primary air inlet from the manifold into the combustion chamber, the primary air inlet being defined between the burner head and an upstream end of the combustion chamber; a secondary air inlet from the manifold into the combustion chamber downstream of the primary air inlet; and means for varying air flow through the primary and secondary air inlets.
More particularly, the combustion chamber comprises first and second portions telescopically movable relative to each other, the secondary air inlet from the manifold into the combustion chamber being defined between the first and second portions of the combustion chamber, the first and second portions of the combustion chamber being relatively movable in a first axial sense to increase air flow through the primary air inlet and reduce air flow through the secondary air inlet, and in a second and opposite axial sense to reduce air flow through the primary air inlet and increase air flow through the secondary air inlet.
It is simplest and most convenient if the first and second portions of the combustion chamber are relatively axially movable so as to vary the air flows through the primary and secondary air inlets in inverse proportion to each other. Preferably, when the air flow through the primary air inlet is at a maximum, the secondary air inlet is fully closed; and when the air flow through the primary air inlet is at a minimum, the secondary air inlet is fully open.
The secondary air inlet may be defined through a wall of the first portion of the combustion chamber. Alternatively, it may be defined through a wall of the second portion of the combustion chamber. As a further alternative, it may be defined through both said walls, e.g., by apertures in both walls moving into or out of registration with each other during relative telescopic movement of the first and second portions of the combustion chamber.
Preferably, the first and second portions of the combustion chamber are respectively movable and fixed with respect to fixed structure of the combustor. Thus, the first (movable) portion of the combustion chamber may be slidable either inside of, or over the outside of, the second (fixed) portion, the movable portion extending upstream such that the primary air inlet is defined between the burner head and an upstream end of the movable portion. Preferably, the first and second portions of the combustion chamber are respectively upstream and downstream portions of the combustion chamber, having only a relatively small mutual overlap sufficient to accommodate the secondary air inlets. Alternatively, but only in the case where the first (movable) portion of the combustion chamber is slidable over the outside of the second (fixed) portion of the combustion chamber, the first and second portions of the combustion chamber overlap over the whole length of the second portion.
Preferably, axial movements in said first and second senses are respectively movements towards and away from the burner head.
An annular seal, such as a piston-ring type seal, is preferably located between the first and the second portions of the combustion chamber to facilitate relative telescopic sliding movement between them.
Conveniently, the telescopic sliding movement may be achieved by connecting the movable portion of the combustion chamber to actuator means for pushing and pulling the movable portion in the first and second axial senses.
The invention further comprises a gas turbine engine provided with at least one gas turbine combustor as described above. In particular, such a gas turbine engine may be provided with at least one combustor in which the actuator is arranged to move the movable portion of the combustion chamber towards the burner head as the engine load decreases, and to move the movable portion of the combustion chamber away from the burner head as the engine load increases.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.